A femtocell will be described in brief.
Femto is a prefix in the metric system, denoting a factor of 10-15. Hence a femtocell or FBS refers to an ultra-small indoor Access Point (AP) for low-power home use or office use. Although the term “femtocell” is sometimes interchangeably used with “picocell”, the former is used in the sense of a more advanced cell. The FBS is a small cellular BS connected to a broadband router and functions to connect 2nd Generation (2G) and 3rd Generation (3G) voice and data to a backbone network of a mobile communication service provider via the Digital Subscriber Line (DSL).
An investigation report was released, saying that FBSs would accelerate 3G proliferation and could be a driving force behind expansion of indoor coverage. It is forecast that by 2011, there will be 102 million users of users of femtocell products and 32 million APs worldwide. According to chief analyst of ABI Research, Stuart Carlaw, “From a technological standpoint, their better in-building coverage for technologies such as WCDMA, HSDPA and EVDO is an incredibly important aspect of service delivery. From a strategic and financial standpoint, the routing of traffic through the IP network significantly enhances network quality and capacity, and reduces the OPEX that carriers expend on backhaul.”
Femtocells can expand cell coverage and increase the quality of voice service. Mobile communication service providers are expecting that subscribers may be familiar with 3G by providing data service via femtocells. The femtocells are also called FBSs or femto Base Transceiver Stations (BTSs).
In summary, femtocells offer the following benefits.
1. Cell coverage improvement
2. Infrastructure cost decrease
3. New service offering
4. Fixed Mobile Convergence (FMC) acceleration.
One or more femtocells may be grouped on a service basis or geological area basis. For example, a femtocell group that allows access to a restricted group of Mobile Stations (MSs) is called a Closed Subscriber Group (CSG). An FBS allows access only to an MS that has subscribed to the CSG by checking the CSG Identifier (ID) of the MS.
FIG. 1 illustrates an exemplary network configuration including FBSs.
FBSs are new network entities added to a legacy network. Accordingly, the use of FBSs may bring about additions or modifications to an entire network structure. An FBS may directly access the Internet and function as a BS. Therefore, the FBS can perform almost all functions of a macro BS. In addition, the FBS may relay data from the macro BS to MSs.
In FIG. 1, the network is configured by adding a Femto Network Gateway (FNG) to the legacy network. The FNG may communicate with an Access Service Network (ASN) gateway and a Connectivity Service Network (CSN). The FNG may use an Rx interface for communicating with the ASN and an Ry interface for communicating with the CSN.
An FBS may access directly a Transmission Control Protocol/Internet Protocol (TCP/IP) Internet and receive a service from the CSN via the FNG. An MS connected to the FBS may receive services from the FNG or CSN in relation to IP Multimedia Subsystem (IMS) authentication, etc.
The FBS is connected to an AP via an R1 interface. This means that the FBS may receive a downlink channel from a macro BS. The FBS may also transmit a control signal to the macro BS.
Femto cells can be classified into the following three types according to a service provided type (i.e., a provided target, e.g., a BS subcarrier type).
1. Open subscriber group femto cell base station (OSG femto cell BS): Every mobile station is granted to receive a service provided by a femto cell.
2. Closed subscriber group open femto cell base station (CSG open femto cell BS): This BS provides all services to a mobile station registered with a closed subscriber group (CSG). Yet, this BS just provides a basic service to a non-registered mobile station.
3. Closed subscriber group open femto cell base station (CSG open femto cell BS): This BS provides a service only to a mobile station registered with a closed subscriber group.
Generally, in a broadband wireless access system, since a number of mobile stations are connected to a macro base station, power consumption of a base station has not been taken into consideration. However, in femto cell supported environment, the number of mobile stations supported by one femtocell is limited (e.g., about 5 to 6 mobile stations). Occasionally, a mobile station maintaining a connection to a femto cell for specific duration may not exist. If the above-described situations repeatedly take place, a femtocell continues to provide a prescribed service to a specific mobile station that does not require the corresponding service. Accordingly, the demand for overcoming the above situations (i.e., femto cell BS power saving) is rising.
Moreover, a mobile station should detect femto cells around to enter a femto cell within a network. Therefore, a method of facilitating a mobile station to detect a femto cell around is necessary.
Besides, a method of reducing interference between a macro cell and a femto cell by turning off the femto cell is necessary as well.